Several types of treatment systems for solid biological waste are presently being developed. These systems should make it possible, firstly to obtain a stabilized and sanitarily reliable end product which can be used for example as compost or soil improver, and secondly to obtain a volume reduction, making dumping less expensive. The types of waste that can be treated using such treatment systems include:
separately collected fruit, vegetable and garden waste; PA1 the organic fraction of household refuse (RDF or wet fraction) from a household refuse treatment plant; PA1 material withdrawn from auctions; PA1 cuttings of verges, trimming waste and the like; PA1 catering waste.
The biological systems can be divided into aerobic and anaerobic systems. An aerobic system is concerned with conventional composting and improvements thereof. Anaerobic systems can be divided into anaerobic fermentation (30.degree.-40.degree. C.), thermophilic fermentation (55.degree.-65.degree. C.) and two-stage systems. Anaerobic fermentation and thermophilic fermentation are processes wherein the entire waste flow is treated, without differentiating between the residence time for liquids and the residence time for solids in the system. Such systems are mostly single-stage and they can be applied both as a completely mixed system or in a plug stream reactor. A disadvantage of these systems is the relatively long treatment time (residence time) and the resulting necessity of large and expensive equipment.
In a two-stage process as defined in the preamble, the selected residence time of the solid material can be longer than the residence time of the liquid and, moreover, different biological reactions can be carried out in different parts of a plant.
A process according to the preamble is known, for example from European patent applications 37612 and 142873. According to EP-A-37612 solid organic material is subjected to a two-stage anaerobic treatment, whereby the material is decomposed into mainly lower fatty acids and other water soluble substances, carbon dioxide, methane and a residual fraction in the first stage, and the water soluble substances and the fatty acids are treated in a second stage leading to formation of methane and carbon dioxide. According to EP-A-142873 the solid material resulting from the first stage (hydrolysis/acidification) is partly recycled and the liquid is treated in a methane reactor in the second stage.
An advantage of these known systems is the relative simplicity of the necessary equipment and of the execution of the process. A disadvantage, however, is that no advanced degree of degradation and thus no substantial volume reduction can be achieved for many types of waste, and thus the cost of dumping is not notably reduced.
A process for the production of methane from solid vegetable waste, wherein the vegetable material is treated with ciliates (microorganisms such as present in the rumen of ruminants) and methane bacteria in a liquid medium, is known from European patent application 159054. The liquid thus obtained can be treated in a separate anaerobic reactor wherein additional methane is produced. Such a method which is thus based on the use of the rumen flora of ruminants, is also known as the so-called RUDAD system (RUmen Derived Anaerobic Digestion).
The RUDAD system has the advantage that waste also containing components that are difficult to degrade can be degraded to a large extent and can thus be considerably reduced in volume. A disadvantage is however that the process should be carefully controlled and that fluctuations of the quantities and nature of the waste supply as occurring in practice disturb the process stability to the extent that an effective waste degradation is no longer achieved.
The problem of fluctations of the supply could in principle be solved by using a large equalizing buffering tank, but this does not solve the problem of rapid acidification of the easily degradable part of the waste (production of acetic and propionic acid). This acidification not only results in stench and lowering of the pH, but also inhibits the biological processes, in particular of the rumen derived flora, when concentrations become excessively high, irrespective of the pH. Such a high concentration of acid components would require a high circulation rate in order to remove all organic acids produced from the rapidly acidifying material; this would results in very short hydraulic retention times of 2 to 4 hours in the RUDAD reactor, whereas a minimum retention time of 6 hours is necessary in order to maintain the desired biomass population, and the optimal hydraulic retention time is about 12 hours. Thus a stable operation of the RUDAD reactor is not possible.